The method of driving fasteners such as nails to secure materials together has changed considerably from the use of manual hammers to the use of rapid action nail driving tools. Such tools, to be fully utilized, require the nails to be collated in a strip or coil whereby many nails can be driven in a short period of time.
Over the past several years nail configurations have been designed to meet specific market needs but many of the configurations, especially the heads, were designed to match the tools. End users prefer a nail with a full round head, since it offers greater holding power due to the larger surface area under the head. This is particularly true where the layer directly under the head is constructed of a soft material such as insulation. In such applications, the shank portion provides very little holding power and the head is the only means that keeps the two workpieces from separating.
The most common method of making round head nails is to grip a single strand of wire in a device with a free portion extending from the clamping jaws. The end of the wire strand is struck by a powered ram. As the free portion is flattened, the wire material spreads outward in a spherical shape.
This flattened portion becomes the head and, at a predetermined distance from the head, the wire is cut to form the workpiece entering end, thus making a finished nail. The cutting device may be constructed to make entering ends of different shapes, e.g., blunt, pointed or chiseled.
To form an acceptable round head, the normal practice is to limit the ratio of the diameter of the head to the diameter of the wire shank to 2.5. Such practice naturally results in nails having small heads with the head size increasing as the wire becomes larger. Making nail heads with a smaller ratio is easy but increasing the ratio creates a number of problems.
To produce a relatively large head, a greater mass of wire is needed and a greater length of wire must extend from the clamping device. When the striking ram hits the end of the extended length of wire, the wire does not always deform equally in every direction. When the mass needed requires the wire to extend a relatively long distance, the extended length may distort as it is being flattened resulting in a head being formed off center of its shank. Although the surface area of such a head may be the same as a centered head and provide equal holding power, problems result when nails with the off-center heads are used in nail driving tools.
Another problem encountered is that the shape of the head may not be round but oval or some other odd shape. Some nail applications require that the shank of the nail be rather stiff to keep it from bending when driven into the workpiece. The wire used to produce such nails must be harder. When such wire is struck to form a head, the flow of material may not be constant resulting in splits around the outside edges of the resultant head.
Most nails are therefore produced with a head to shank diameter ratio of 2.5 or less to avoid off-center and/or non-round heads. "Roofing nails" deviate from this ratio, having a shank diameter of 3.1 mm with a head diameter of 10 mm. To produce "roofing nails" on standard nail making machinery requires that the wire used has a very low carbon content to minimize the problems described above. A head of such material is usually thin and will bend quite easily since the wire used is softer.
After the nails are produced, they are then collated in an assembly that is adapted to function in a particular powered nailing device. The assembly may be in the form of a strip of nails or rolled into a coil. The coil shaped assemblies are preferred since a larger quantity of nails can be placed in a nail driving device, assuming that the same size of tool is used for both strip and coil assemblies.
Although making the headed nail first and then collating them in an assembly is the most common practice used, U.S. Pat. No. 5,140,715 (assigned to the assignee of this invention) teaches an alternate method. This method of production is a continuous process starting with a strand of wire and ending with an assembly of headed nails. The head is formed on a series of headless nails after they have been collated in a continuous web. This method is an improvement over previous teachings, but it too places restrictions on the finished product. Since the head is formed by a punch the head diameter cannot be greater than the spacing between the headless nails of the continuous web. By making the web with off setting headless nails, a larger head could be formed if the punch struck only one nail at a time. But this would defeat the cost saving feature which results from the production method of U.S. Pat. No. 5,140,715.
Even if the larger heads could be formed, the disadvantages or restrictions in producing large head nails first and then collating would still exist. Applications for large head nails have increased, many of which cannot accept the restrictions placed upon the finished nail by present nail producing methods. One solution to the problem is to first produce a standard nail and insert it into a washer prior to driving. This can be done as two separate parts during the driving process or have the washer attached to the nail previously. In either case, the cost of the final product increases considerably.
Variations of nail strips are shown in U.K. patent nos. 1,348,703 and 1,427,128. U.S. Pat. Nos. 2,784,405 and 3,506,115 show nails inserted in a carrier strip that is sheared during the driving operation. Although the material under the head remains with the nail, the purpose of the strip is to allow automatic feeding of nails by the nail driving tool. All of these mentioned patents teach the process of producing the assembly of nails starting with a headed nail which is inserted through a hole in a strip of metal to produce the nail assembly.